This invention relates to hydrostatic transmissions. More specifically this invention relates to a hydrostatic transmission that utilizes a proportional pressure variable displacement hydraulic pump for both loop charge and fan flow supply.
Current hydrostatic transmissions combine supply flow and make-up flow from a single charge pump. In these circuits, a fixed displacement charge pump provides flow to the hydraulic motor that powers the cooling fan. The return oil leaving the hydraulic motor is then routed to the charge flow supply port of the hydrostatic transmission.
The supply port is connected to a “charge relief valve” which limits the charge supply pressure and limits the back pressure on the hydraulic fan motor. The speed of the hydraulic motor is regulated by controlling the by-pass pressure across the motor by the use of the relief valve that is plumbed in a parallel arrangement with the hydraulic motor. The relief valve may be either a fixed or a proportional pressure setting.
When a fixed pressure setting is used, the fixed pressure setting limits the maximum speed of the hydraulic fan motor. Alternatively when a proportional pressure setting is provided the proportional pressure setting allows the hydraulic fan motor speed to be controlled, or limited to meet the requirements of the cooling system.
Pending global emissions regulations have increased the need to minimize parasitic losses in vehicles and systems that use hydrostatic transmissions for propel and work functions. One method of reducing parasitic losses in these vehicles is to replace the fixed displacement charge pumps with variable displacement pumps. This is especially true when these pumps are used to supply flow to make up the charge flow requirements of the system and to drive the system's hydraulic cooling fans.
Typically, the pressure required to drive the cooling fan is several times the requirement of the “charge” circuit. Therefore present hydrostatic transmissions commonly by-pass fluid around the fan motor to regulate the delta pressure across the motor and/or regulate the speed of the fan motor. Consequently, any additional flow that is by-passed across a pressure regulating device, or relief valve, only contributes to increased parasitic losses and/or heat that must be removed from the system.
Therefore a principal object of the present invention is to provide a hydrostatic transmission that minimizes parasitic losses.
Yet another object of the present invention is to provide a hydrostatic transmission that minimizes the heat that must be removed from the transmission.
These and other objects, features, and advantages of the present invention will be discussed in the specification and claims.